In the markets, many conventional printers (e.g. label printers) use paper rolls as print media. Generally, a paper roll of the label printer comprises a hollow shaft and a paper tape wound on the hollow shaft. The paper tape comprises a laminated film strip and plural label papers. The label papers are discretely attached on the laminated film strip at regular intervals. During the printing task is performed, the paper tape is driven to be moved through a printing mechanism, and the label papers are printed by the printing mechanism. Moreover, during the printing task is performed, the label papers are successively outputted from the label printer. Correspondingly, the paper roll is rotated, and thus the overall diameter of the paper roll is gradually decreased. If the paper roll is moved up and down or shifted left and right upon rotation of the paper roll, the rotating and printing smoothness are impaired. For enhancing the rotating and printing smoothness, a fixing structure or a supporting structure is installed within the label printer for preventing the paper roll from being moved up and down or shifted left and right upon rotation of the paper roll.
FIG. 1 is a schematic exploded view illustrating a conventional label printer. The components of the conventional label printer 1 will be illustrated as follows. As shown in FIG. 1, the conventional label printer 1 comprises a paper roll fixing device 11 and a casing 12. The paper roll fixing device 11 is disposed within the casing 12.
The paper roll fixing device 11 comprises a first supporting member 111, a second supporting member 112, a first movable track 113, a second movable track 114, and a rotating wheel 115. The first supporting member 111 comprises a first supporting protrusion part 111a. The second supporting member 112 comprises a second supporting protrusion part (not shown). The second supporting protrusion part has the same structure as the first supporting protrusion part 111a. 
Moreover, the first supporting protrusion part 111a comprises a first ring-shaped structure 111aa, a second ring-shaped structure 111ab and a third ring-shaped structure 111ac, which have different outer circumferences. The second supporting protrusion part comprises a fourth ring-shaped structure, a fifth ring-shaped structure and a sixth ring-shaped structure (not shown). The outer circumferences of the fourth ring-shaped structure, the fifth ring-shaped structure and the sixth ring-shaped structure are identical to the outer circumferences of the first ring-shaped structure 111aa, the second ring-shaped structure 111ab and the third ring-shaped structure 111ac, respectively. The outer circumference of the first ring-shaped structure 111aa is smaller than the outer circumference of the second ring-shaped structure 111ab. The outer circumference of the second ring-shaped structure 111ab is smaller than the outer circumference of the third ring-shaped structure 111ac. 
Hereinafter, a sequence of assembling the conventional label printer 1 will be illustrated with reference to FIG. 1. Firstly, an end 113a of the first movable track 113 is connected with the first supporting member 111, and an end 114a of the second movable track 114 is connected with the second supporting member 112. Then, the first movable track 113 and the second movable track 114 are parallel with each other, and the rotating wheel 115 is clamped between and engaged with the first movable track 113 and the second movable track 114.
In addition, the first supporting member 111 and the second supporting member 112 are parallel with each other, and the first supporting protrusion part 111a and the second supporting protrusion part face each other.
The operations of the conventional label printer 1 will be illustrated in more details as follows. Please refer to FIG. 1 again. As mentioned above, the paper roll comprises a hollow shaft and a paper tape wound on the hollow shaft. For installing the paper roll within the label printer 1, the first supporting member 111 or the second supporting member 112 is firstly pulled. Through the rotating wheel 115, the first movable track 113 and the second movable track 114 may be linearly moved. Consequently, the first supporting member 111 and the second supporting member 112 are distant from each other. Then, a first end of the hollow shaft of the paper roll is fixed by the first supporting protrusion part 111a of the first supporting member 111.
Then, the second supporting member 112 is moved. During the process of moving the second supporting member 112, the first supporting member 111 and the second supporting member 112 are moved toward each other through the rotating wheel 115. Until a second end of the hollow shaft of the paper roll is fixed by the second supporting protrusion part of the second supporting member 112, the paper roll is fixed within the label printer 1.
If the inner circumference of the hollow shaft is equal to the outer circumference of the first ring-shaped structure 111aa, the first ring-shaped structure 111aa is inserted into the first end of the hollow shaft of the paper roll, and the fourth ring-shaped structure is inserted into the second end of the hollow shaft of the paper roll. In addition, the first ring-shaped structure 111 as and the fourth ring-shaped structure are contacted with the inner peripheries of the hollow shaft of the paper roll in order to fix the position of the paper roll.
Similarly, if the inner circumference of the hollow shaft is equal to the outer circumference of the second ring-shaped structure 111ab, the second ring-shaped structure 111ab and the fifth ring-shaped structure are respectively inserted into the first end and the second end of the hollow shaft of the paper roll. Similarly, if the inner circumference of the hollow shaft is equal to the outer circumference of the third ring-shaped structure 111ac, the third ring-shaped structure 111ac and the sixth ring-shaped structure are respectively inserted into the first end and the second end of the hollow shaft of the paper roll.
From the above discussions, the ring-shaped structures of the conventional label printer 1 are inserted into the two ends of the hollow shaft of the paper roll, so that the position of the paper roll is fixed.
However, the conventional label printer 1 still has some drawbacks. The inner circumferences of the hollow shafts of the paper rolls from different factories may be different. Regardless of the inner circumferences of the hollow shafts, the widths of these paper rolls are identical. In a case that the paper roll fixing device is equipped with plural ring-shaped structures matching all paper roll specifications, the paper roll fixing device may occupy much space and thus the volume of the casing will be very large.
In a case that the plural ring-shaped structures only match some paper roll specifications, the applications of the paper roll fixing device are restricted. For example, if the inner circumference of the hollow shaft of the paper roll is greater than the largest outer circumference of the plural ring-shaped structures, the paper roll is moved downwardly in response to the gravity force thereof. Under this circumstance, the casing 12 is pressed by the bottom of the paper roll.
In other words, although the paper roll is not shifted left and right during the printing task is performed, the paper tape is readily rubbed against the casing 12 upon the rotation of the paper roll. Consequently, the paper tape fails to be smoothly moved, and the possibility of abrading or stripping the label papers is increased.
Therefore, there is a need of providing an improved paper roll fixing device of a label printer in order to eliminate the above problems.